Direct start 10 HP motor - Faulty contactor
Direct start 10 HP motor - Faulty contactor
(OP)
Dear Experts
We have in operation a machine that has a 10 HP electric motor – 3x380 V – 50 Hz
The motor has direct start with a contactor. The problem is this:
Every 2 month (aprox) there is a fault in the contactor. Most of the time the contacts melt and weld together. This is a happening for a long time and what we do is have a spare contactor and change it and then wait for the next 2 month.
The nature of the work made by the machine is so that the operator makes many starts and stops throught out the day on this motor
How can I do to finally solve this problem? Should I used an over rated concator, for example 20 HP?
Thanks
We have in operation a machine that has a 10 HP electric motor – 3x380 V – 50 Hz
The motor has direct start with a contactor. The problem is this:
Every 2 month (aprox) there is a fault in the contactor. Most of the time the contacts melt and weld together. This is a happening for a long time and what we do is have a spare contactor and change it and then wait for the next 2 month.
The nature of the work made by the machine is so that the operator makes many starts and stops throught out the day on this motor
How can I do to finally solve this problem? Should I used an over rated concator, for example 20 HP?
Thanks





RE: Direct start 10 HP motor - Faulty contactor
So yes, you most likely have to increase the size of that contactor. Every contactor manufacturer has a chart available that is used to plot the current, duty cycle and expected lifespan of their products. Elicit the help of your manufacturer in selecting the proper size for your application.
That said, another cause of rapid failure is sometimes attributable to faulty control circuit issues causing what is called "chatter" in the contactor. For example a low control voltage can cause the contactor to not hold in properly, or a bad connection on a contact block in a control element such as a push button or relay can cause intermittent operation. Check those things fully before jumping into a new contactor, because if a problem like that exists you will smoke the new contactor too, it just may take a little longer.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Direct start 10 HP motor - Faulty contactor
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Direct start 10 HP motor - Faulty contactor
Keith Cress
kcress - http://www.flaminsystems.com
RE: Direct start 10 HP motor - Faulty contactor
Should I install some kind of timer to prevent restarting?
RE: Direct start 10 HP motor - Faulty contactor
You might consider choosing your contactor based on its AC-4 rating rather than the normally quoted AC-3 rating. AC-4 is a severe duty rating for plugging and reversing applications: normally it effectively results in using a contactor one frame size larger.
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If we learn from our mistakes I'm getting a great education!
RE: Direct start 10 HP motor - Faulty contactor
Model: CMW 25
RE: Direct start 10 HP motor - Faulty contactor
Yes timers would be a good solution as they would also remove switch chatter or hesitation.
You would want a timer to let the motor reach speed before turn off and ideally some sort of time to prevent turning the motor on more frequently than absolutely necessary.
Perhaps the process doesn't need to be turned on and off more often than every few minutes, most motors that size would not like to be turned on/off more than ~ 20 times an hour, so then set the on relay to a few minutes.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Direct start 10 HP motor - Faulty contactor
Maximum ops/hr = 1200 at AC-3 but drops to 360 at AC-4.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Direct start 10 HP motor - Faulty contactor
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Direct start 10 HP motor - Faulty contactor
Constant jogging with the starter has to be avoided if possible. It could involve both opening under starting surge amperage as well as restarting out of phase with the residual magnetic field in the motor. Or not.....the difference between basically operating with the contacts chattering and just concern about the number of starts per hour is only a matter of seconds.
On the other hand, a new contactor every 2 months might be the cheapest solution.
RE: Direct start 10 HP motor - Faulty contactor
So, that is why I want to electricaly protect the contactor.
I understand that the reasons the contactor fails are 2:
1. opening contactor before it finishes starting the motor.
2. Re-closing with residual current
Can anyone recommend an instalation scheme? drawings? to avoid these
Thanks
RE: Direct start 10 HP motor - Faulty contactor
the cost of one contactor is: 32 USD
RE: Direct start 10 HP motor - Faulty contactor
Changing the contactor every 60 days may be a reasonable price to avoid the complaining that will almost certainly result from making a change to the machines operational characteristics.
RE: Direct start 10 HP motor - Faulty contactor
One out of four had multiple contactor issues. We finally replaced every contactor in the control box with Allen Bradley contactors and never had another contactor failure. We lost a lot of work. grin.
You may want to consider an Allen Bradley, size 2, 25 HP@380V, Part number 509-CO* (*=Coil voltage).
The choice between Eutectic melting alloy O/Ls and electronic O/L protection is another discussion. Either will work. Eutectic has no differential protection but has very good over current protection.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Direct start 10 HP motor - Faulty contactor
Don't be frightened by list prices in a fairly new catalog. Catalog list prices must be used with a discount schedule. There may be a 4:1 ratio between list prices and actual cost.
I have seen a $20,000 quote at replacement prices drop to $5000 at competitive bid prices.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Direct start 10 HP motor - Faulty contactor
David Castor
www.cvoes.com
RE: Direct start 10 HP motor - Faulty contactor
By this I meant that three cranes worked well (and most of the other cranes that I saw only once or twice.) One crane had undersized, poor quality contactors and in damp weather we had a failure and trouble call once a week or more. That's a lot of down time for a working truck loading yard. Good quality contactors, properly sized solved the issues.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Direct start 10 HP motor - Faulty contactor
I have 3 options here:
Telemecanique
Siemens
Moeller
Allen Bradley: I Did not find anyone who sells this brand here
I am writing from Paraguay
RE: Direct start 10 HP motor - Faulty contactor
Size the contactor from the AC-4 ratings, not AC-3. That might well push you up one frame size compared with AC-3, and you'll end up with a contactor roughly the same size as the NEMA equivalent.
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If we learn from our mistakes I'm getting a great education!